What does the defrost function change in a microwave?

[marmaduke]

In a microwave, what does the defrost button change or do differently?

[5029]

It uses a lower heat setting (usually about 40%) so that the food defrosts uniformly, instead of ending up cooked on the outside, frozen on the inside

[Clanad]

According to How Things Work...

During the defrost cycle, the microwave oven periodically turns off its magnetron so that heat can diffuse through the food naturally, from hot spots to cold spots. These quiet periods allow frozen parts of the food to melt the same way an ice cube would melt if you threw it into hot water. While the magnetron is off, it isn't emitting any microwaves and the food is just sitting there spreading its thermal energy around.

[Gef]

When the magnertron is on it always delivers the same power. As Clanad says anything other than full power uses a timer to switch the microwaves on or off. This is why it is often difficult to overheating some parts of the food. 40 secs on medium might mean 10 secs on, 10 off, 10 on and 10 off. 30 secs on full would be the same 10 on, 10 off, 10 on. Hope you understand this.